He cited a young private who was sitting in his pickup in a parking lot and heard gunfire. Casey said the private went to the scene, pulled out four of the wounded and drove to the hospital.

Moment of silenceMilitary, friends and families observed a moment of silence at U.S. military bases around the world as a show of respect for the victims. Dozens of people gathered at Fort Hood and bowed their heads as part of the ceremony.

Earlier in the day, President Barack Obama said the "entire nation is grieving" over the mass killings. He urged people not to jump to conclusions while law enforcement officials gather facts about the shootings.

Obama met Friday morning with FBI Director Robert Mueller and other federal leaders to get an update on what they've learned.

"We don't know all the answers yet. And I would caution against jumping to conclusions until we have all the facts," Obama said in a Rose Garden statement otherwise devoted to the economy.

"What we do know is that there are families, friends and an entire nation grieving right now for the valiant men and women who came under attack yesterday," the president said.

'Awe of their sacrifice'
Obama ordered the flags at the White House and other federal buildings to be at half-staff until Veterans Day. He called it a modest tribute to those who were slain and to those who put their lives on the line in the armed services each day.

"We stand in awe of their sacrifice, and we pray for the safety of those who fight, and for the families of those who have fallen," Obama said.

He promised that his administration would update the nation as it learns more about what happened, and why, at Fort Hood.

Meanwhile, military officials were starting to piece together what may have pushed an Army psychiatrist trained to help soldiers in distress to turn on his comrades.

A doctor at the hospital where several of the injured were taken said some patients may die. Dr. W. Roy Smythe said some of the wounded had "extremely serious injuries."

At least 30 people were injured. Some sources said that the number could be as high as 43 if non-gunshot wounds and trauma were counted.

Suspect hospitalizedThe suspected shooter, Maj. Nidal Malik Hasan, was on a ventilator and unconscious after being shot four times during the shootings, Fort Hood officials said. In the early chaos after the shootings, authorities believed they had killed him, only to discover later that he had survived.

In Washington, a senior U.S. official said authorities at Fort Hood initially thought one of the victims who had been shot and killed was the shooter. The mistake resulted in a delay of several hours in identifying Hasan as the alleged assailant.

The commander of the Fort Hood, Lt. Gen. Bob Cone, told NBC's TODAY on Friday that Hasan was in a "stable condition." He said he would be interrogated as soon as possible.

Cone also said he heard first-hand accounts from witnesses on the scene that the suspect shouted "Allahu Akbar," which means "God is Great" in Arabic, before he opened fire at the Soldier Readiness Center at Fort Hood.

In a separate interview, Cone said survivors have told him the shooter carried out his gunfire in "a very calm and measured approach."

Video: What did Hasan say before rampage?
Some 300 soldiers were lined up to get shots and eye-testing when the shots rang out. Cone said one soldier who had been shot told him, 'I made the mistake of moving and I was shot again.' "

The general said survivors told him that during the rampage, soldiers "would scramble to the ground and help each other out." Cone appeared on CBS's "The Early Show."

'Friendly fire'
Authorities have not ruled out that Hasan was acting on behalf of some unidentified radical group, the official said. He would not say whether any evidence had come to light to support that theory.

The official spoke to The Associated Press on condition of anonymity to discuss matters that were under investigation.

Officials are not ruling out the possibility that some of the casualties may have been victims of "friendly fire," that in the mayhem and confusion at the shooting scene some of the responding military officials may have shot some of the victims.

The gunfire broke out around 1:30 p.m. at the Soldier Readiness Center, where soldiers who are about to be deployed or who are returning undergo medical screening. Nearby, some soldiers were readying to head into a graduation ceremony for troops and families who had recently earned degrees.

‘Sir, they are opening fire over there!’Pastor Greg Schannep had just parked his car along the side of the theater and was about to head into the ceremony when a man in uniform approached him.

"Sir, they are opening fire over there!" the man told him. At first, he thought it was a training exercise — then heard three volleys and saw people running. As the man who warned him about the shots ran away, he could see the man's back was bloodied from a wound.

cstsonline.org

Maj. Nidal Malik Hasan was described as ‘upset’ about his pending deployment.

Schannep said police and medical and other emergency personnel were on the scene in an instant, telling people to get inside the theater. The post went into lockdown while a search began for a suspect and emergency workers began trying to treat the wounded. Some soldiers rushed to treat their injured colleagues by ripping their uniforms into makeshift bandages to treat their wounds.

Cone made special mention of Amber Bahr, 19, an army nutritionist who was wounded during the attack. He said she helped wounded soldiers during the rampage. Only after the attack did she realize that she herself was wounded, Cone said.

Her mother, Lisa Pfund, told the Sheboygan Press that she spoke briefly to Bahr after she was taken to a community hospital.

"I actually got to talk to Amber and I talked to her for about 30 seconds and she was in a lot of pain," Pfund said. "She couldn't tell me nothing, either."

ShockVideo from the scene showed police patrolling the area with handguns and rifles, ducking behind buildings for cover. Sirens could be heard wailing while a woman's voice on a public-address system urged people to take cover. Schools on the base went into lockdown, and family members trying to find out what was happening inside found cell phone lines jammed or busy.

"I was confused and just shocked," said Spc. Jerry Richard, 27, who works at the center but was not on duty during the shooting. "Overseas you are ready for it. But here you can't even defend yourself."

The wounded were dispersed among hospitals in central Texas, Cone said. Their identities and the identities of the dead were not immediately released.

The bodies of the victims would be taken to Dover Air Force Base in Delaware for autopsies and forensic tests, said a U.S. official who spoke on condition of anonymity to discuss matters that were under investigation.

There will also be a ceremony at the air base to honor the dead.

Jamie and Scotty Casteel stood outside the emergency room at the hospital in Temple waiting for news of their son-in-law Matthew Cooke, who was among the injured.

"He's been shot in the abdomen and that's all we know," Jamie Casteel told The Associated Press. She said Cook, from New York state, had been home from Iraq for about a year.

Ashley Saucedo told WOOD-TV in Michigan that her husband was shot in the arm, but she couldn't discuss specifics. Saucedo said she and the couple's two children weren't permitted to leave their home at Fort Hood during the shootings.

The motive for the shooting wasn't clear, but Hasan was apparently set to deploy soon, and had expressed some anger about the wars in Iraq and Afghanistan.

Army spokeswoman Col. Cathy Abbott said Hasan had been scheduled to deploy to Afghanistan, and not Iraq, to counsel soldiers suffering from combat stress.

Hasan, an active-duty military psychiatrist, was to deploy with an Army Reserve unit that provides what the military calls "behavioral health" counseling, Abbott said. It wasn't immediately clear whether Hasan sought the assignment or was being sent against his wishes.

Retired Col. Terry Lee said Hasan had hoped Obama would pull troops out of Afghanistan and Iraq and got into frequent arguments with others in the military who supported the wars.

For six years before reporting for duty at Fort Hood, in July, the 39-year-old Army major worked at the Walter Reed Army Medical Center pursuing a career in psychiatry, as an intern, a resident and, last year, a fellow in disaster and preventive psychiatry. He received his medical degree from the military's Uniformed Services University of the Health Sciences in Bethesda, Md., in 2001.

But his record wasn't sterling. At Walter Reed, he received a poor performance evaluation, according to an official who spoke on condition of anonymity because he was not authorized to discuss the case publicly. And while he was an intern, Hasan had some "difficulties" that required counseling and extra supervision, said Dr. Thomas Grieger, who was the training director at the time.

At least six months ago, Hasan came to the attention of law enforcement officials because of Internet postings about suicide bombings and other threats, including posts that equated suicide bombers to soldiers who throw themselves on a grenade to save the lives of their comrades.

Investigators had not determined for certain whether Hasan was the author of the posting, and a formal investigation had not been opened before the shooting, said law enforcement officials who spoke to The AP on condition of anonymity because they are not authorized to discuss the case.

Suspect emptied apartment
A neighbor said Hasan cleaned out his apartment in the days before the rampage.

Neighbor Patricia Villa said Hasan came over to her apartment on Wednesday morning and told her he was going to be deployed on Friday.

She said he gave her some frozen broccoli, some spinach, T-shirts, shelves and a new Quran, the Muslim holy book. She said he returned on Thursday morning and gave her his air mattress, several briefcases and a desk lamp.

Villa said Hasan then offered her $60 to clean his apartment Friday morning after he supposedly was to leave.

The FBI, local police and other agencies searched Hasan's apartment Thursday night after evacuating the complex in Killeen, said city spokeswoman Hilary Shine. She referred questions about what was found to the FBI. The FBI in Dallas referred questions to a spokesman who was not immediately available early Friday morning.

Video: Soldier thought shooting was 'training'

Closed captioning of: Soldier thought shooting was 'training'

>>>good evening. as the nation's largest military post deals with the concussion of what happened there yesterday, the colossal loss of life, tonight a single gunman is hospitalized under heavy guard, and the numbers have gone up to 13 killed, 43 injured in that single attack on ft. hood in texas. and a new picture is emerging of this accused gunman literally and figuratively. the army major, a trained psychiatrist. as we now know much more about his past and his
personal life
. again, tonight we have all of it covered, beginning with
lester holt
in ft. hood tonight. lester, good evening.

>>reporter: brian, good evening. the accused shooter, who himself was shot and wounded, has been transferred to the brook
army medical center
in
san antonio
. that's about 150 miles from here. he is in stable condition in the intensive care unit there. meantime, his former comrades back here are left to wonder why and also to wonder whether anyone could have or should have seen this coming. the army's top brass at ft. hood today leading soldiers in a moment of silence, exactly
24 hours
after one of their own turned this stateside post into a battleground.

>>i'll tell you candidly, this was a kick in the gut. not only for the ft. hood community but also for our entire army.

>>reporter: officials today confirmed the carnage was the work of one man, army major and psychiatrist nidal
hasan
, who they say turned on the very soldiers he was charged with caring for.

>>people were shot just about everywhere. certainly it's hard to imagine one person doing this much damage.

>>reporter: the picture emerging of
hasan
is of a man increasingly anguished over his imminent deployment to afghanistan. still, they say, there was no warning when at
1:34 p.m
.
hasan
, dressed in his army uniform, opened fire inside the fort's soldier readiness center, where men and women preparing for deployment were undergoing medical screenings. also in the line of fire, a group filing into a graduation ceremony. army photographer specialist elliott valdez was hitting into the theater and told his story to a military video crew.

>>people outside of the theater coming inside screaming. there had been one person that made his way to the theater that had been shot. since that happened, i just grabbed my camera and just went outside.

>>reporter: in an exclusive interview with nbc's
salt lake city
affiliate ksl a father of a soldier whose identity he did not want revealed described what his daughter says
happened before
the gunman started shooting.

>>he said
allah akbar
and just opened up on everybody. and initially she thought that it was part of the training some real-life scenario, but it was within a second or two that she realized it was for real because the person next to her was shot and killed instantly.

>>your primary -- secondary concern is just for your family. so you're not feeling that
ill will
or hard feelings against someone that would -- you're just questioning why. why would anyone in their right mind start something like this.

>>reporter: this morning
president obama
said the motive of the attack is still unknown.

>>what we do know is that there are families, friends, and an entire nation grieving right now for the valiant men and women who came under attack yesterday.

>>reporter: army officials are for now treating the attack as an isolated incident and cautioning against a rush to judgment.

>>i do worry slightly about a potential backlash.

>>reporter: with 13 dead and over 40 injured, the army hopes the attack won't create deeper wounds within the brotherhood of soldiers. in one of the many sad ironies of this tragedy, the bodies of those who were killed here are making the same solemn journey tonight that america's war dead do. they are being flown, brian, to
dover air force base
in delaware to a military mortuary, where they will eventually be turned over to their families.

>>lester holt
starting us off tonight from the main gate there at ft. hood in texas. lester, thanks.